1. Field of the Invention
The present invention relates to a brushless motor that has a skew structure. More particularly, the invention relates to a brushless motor having a step-skew structure by means of segment magnets.
2. Related Art Statement
Hitherto known, as means for reducing cogging torque and torque ripple, is the skew structure in which the rotor poles or the like are inclined to the axis direction. In most brushless motors of skew structure, ring magnets are used as pole magnets. In any motor using ring magnets, the magnets are skew-magnetized in order to reduce cogging torque and torque ripple.
In brushless motors designed for use in electric power steering apparatuses, the segment magnets that can be magnetized at high flux density are increasingly used as rotor magnets, thereby to meet the demand that the motors should be smaller and should yet achieve a large output. However, the segment magnets (right-angled magnetic field type magnets) cannot be skew-magnetized due to the restriction imposed on their manufacture. In any motor that has segment magnets, the segment magnets are therefore stacked one on another, thus achieving a so-called step skew in order to perform skew structure.
In the motor of step-skew structure, the segment magnets are arranged in even-number stages (usually, two stages), in the axial direction in order to offset the cogging waves at the respective steps to reduce the cogging torque. Patent Document 1 discloses a rotating electrical machine in which magnets are arranged in two stages. In the rotating electrical machine disclosed in the patent, the magnets of each stage are arranged in the circumferential direction, each magnet being shifted at a specific angle from the next. The poles of the rotor are thereby displaced stepwise in the axial direction, whereby a two-staged, step-skew structure is constructed.
In the step-skew structure, however, assembled state, physical properties, process precision, and the like actually have a variation from the design specification. Inevitably, the cogging torque cannot be reduced so much in the two-staged, step-skew structure. The cogging torque can hardly be reduced, particularly in any motor that has a few poles and a few slots because the least common multiple of the number of poles and the number of slots is small, inevitably increasing the influence of the variation. In view of this, the accuracy of the magnets and parts assembling are very strict, and the motor cannot be as robust as desired.
A six-pole, nine-slot motor, for example, exhibits a ninth-order cogging waveform (for every rotation) because of the variation in terms of rotor manufacture precision, and exhibits sixth-order cogging waveform (for every rotation) because of the stator variation. The fundamental cogging waveform is therefore the 18th-order waveform (that is, the least common multiple of 9 and 6). The cogging waveform attributable to the assembly unbalance is an even-number times each order of the cogging waveform. As a result, an integral multiple of each order will be added to the cogging. Particularly, the variation of the rotor in manufacture precision caused by the stepped structure greatly contributes to the cogging. If the 36th-order component of cogging waveform caused by the stepped structure variation becomes prominent, the cogging cannot be reduced even at the logical skew angle (360°/the least common multiple of poles and slots).
In order to reduce both the cogging and the torque ripple, when the magnets must have a large width and a large skew angle to reduce the harmonic components of the induced voltage, any two adjacent poles (different in polarity) of each stage may overlap each other. In case the magnets having different polarity overlap each other, the magnetic fluxes will be cut short. As a result, effective fluxes required for driving the motor will not be provided. Consequently, the skew angle will not be increased, and so the cogging torque and the like cannot be reduced enough, and the effective fluxes decrease to result in a small motor output.